Since a hydrogen storage alloy is an alloy that reacts with hydrogen to form a metal hydride, and reversibly absorb and desorb a large quantity of hydrogen around room temperature, the actual application has been studied in various fields, such as a nickel-hydrogen battery used in hybrid vehicles or digital still cameras.
As a hydrogen storage alloy, various alloys have been known, such as an AB5-type alloy represented by LaNi5 and an AB2-type alloy represented by ZrV0.4Ni1.5, as well as an AB-type alloy and an A2B-type alloy. Many of these are composed of a combination of an element group having a high affinity with hydrogen and a large hydrogen storage capacity (Ca, Mg, rare-earth elements, Ti, Zr, V, Nb, Pt, Pd, etc.), and an element group having a relatively low affinity with hydrogen and a small hydrogen storage capacity, but having a high hydrogenation reaction rate and lowering the reaction temperature (Ni, Mn, Cr, Fe, etc.). Since any types of these alloys largely vary the characteristics depending on the composition, various alloy compositions have been studied with the objectives of improving the maximum hydrogen storage capacity and effective hydrogen storage capacity (increase of capacity), prolonging the life performance, and obtaining higher output.
Among them, the study group of the present inventors has focused attention on and studied an AB5-type hydrogen storage alloy having a CaCu5-type crystal structure, specifically Mm-Ni—Mn—Al—Co alloy consisting of Mm (Misch metal), which is a rare-earth mixture, in the A site, and four elements of Ni, Al, Mn and Co in the B site. This type of Mm-Ni—Mn—Al—Co alloy has features that can compose a negative electrode using relatively inexpensive materials compared with La-based alloys, and can obtain a closed-type nickel-hydrogen secondary battery with long cycle life and little internal pressure elevation due to gases generated in overcharging.
Meanwhile, in the composing elements of the Mm-Ni—Mn—Al—Co alloy, since Co is an important element to suppress pulverization of the alloy, and to exert the effect of improving life performance, heretofore, about 10% by weight of Co (molar ratio: 0.6 to 1.0) was generally and conventionally compounded. However, Co is a very expensive metal, and it is preferable to reduce Co when taking account of the future expansion of use of hydrogen storage alloys. However, the reduction of Co leads to lowering of drain (power) performance and life performance; it has therefore been a research project to reduce Co while maintaining drain (power) performance and life performance. Particularly, in order to develop the application of hydrogen storage alloy into a power source and the like for electric vehicles (EV) and hybrid electric vehicles (HEV, motor vehicles using two power sources of electric motors and internal combustion engines), it has been an essential challenge to maintain drain (power) performance and life performance at high levels.
In view of these problems, various proposals to reduce Co quantities and still maintain the cell performance have been disclosed.
For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 9-213319) proposes to change the composition of Mm-Ni—Mn—Al—Co-based alloy, and further add a small quantity of a single element to the alloy.
Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-294373) proposes a hydrogen storage alloy having the composition of Equation (1), of substantially single phase, wherein the average major axis of the crystal is 30 to 160 μm, or 5 μm to less than 30 μm.RNixCoyMz  (1)(R: rare-earth element or the like; M: Mg, Al, Mn or the like; 3.7≦x≦5.3, 0.1≦y≦0.5, 0.1≦z≦1.0, 5.1≦x+y+z≦5.5)
The study group that the present inventors belong also proposes, for example in Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-18176), a hydrogen storage alloy having a CaCu5-type crystal structure represented by general formula MmNiaMnbCocCud, where Mm is a Misch metal, 3.7≦a≦4.2, 0.3≦b≦0.6, 0.2≦c≦0.4, 0<d≦0.4, 5.00≦a+b+c+d≦5.35, as a hydrogen storage alloy that has a reduced cobalt content, has excellent hydrogen storage capacity, has favorable pulverization performance, initial performance and drain (power) performance, and has high reliability in durability and storage stability.
In Patent Document 4 (Japanese Patent Application Laid-Open No. 2001-40442), the study group proposes a hydrogen storage alloy having a CaCu5-type crystal structure represented by general formula MmNiaMnbAlcCodXe (Mm being a Misch metal, X being Fe and/or Cu, 3.7≦a≦4.2, 0≦b≦0.3, 0≦c≦0.4, 0.2≦d≦0.4, 0≦e≦0.4, 5.00≦a+b+c+d+e≦5.20, provided that b and c are not simultaneously 0, and when 0<b≦0.3 and 0<c≦0.4, b+c<0.5) as a hydrogen storage alloy wherein the cobalt content is reduced, having excellent hydrogen storage capacity, having pulverization performance, favorable initial performance and drain (power) performance, and having high reliability in durability and storage stability.
In Patent Document 5 (Japanese Patent Application Laid-Open No. 2001-348636) the study group proposes an AB5-type hydrogen storage alloy having a CaCu5-type crystal structure represented by the general formula MmNiaMnbAlcCod, where Mm is a Misch metal, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, 5.2≦a+b+c+d≦5.45), or represented by the general formula MmNiaMnbAlcCodXe (where Mm is a Misch metal, X is Cu and/or Fe, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, 0<e≦0.1, 5.2≦a+b+c+d+e≦5.45, as a hydrogen storage alloy that is produced at a reduced cost by extremely reducing the cobalt content, has excellent pulverization performance and hydrogen storage capacity, and has favorable drain (power) performance and storage stability.    Patent Document 1: Japanese Patent Application Laid-Open No. 9-213319    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-294373    Patent Document 3: Japanese Patent Application Laid-Open No. 2001-18176    Patent Document 4: Japanese Patent Application Laid-Open No. 2001-40442    Patent Document 5: Japanese Patent Application Laid-Open No. 2001-348636